The present embodiments relate to communication systems and, more particularly, to opportunistic Intermediate Frequency (IF) selection to reduce Adjacent Channel Interference (ACI).
New standards for Digital Video Broadcast (DVB) standards are currently being developed to permit streaming video reception by portable user equipment. DVB typically uses carrier frequencies in the 470-800 MHz band. DVB packets or data streams are transmitted by Orthogonal Frequency Division Multiplex (OFDM) transmission with time slicing. With OFDM, multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is considered as frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver, and these tones are termed pilot tones or symbols. These pilot symbols can be useful for channel estimation at the receiver. An inverse fast Fourier transform (IFFT) converts the frequency domain data symbols into a time domain waveform. The IFFT structure allows the frequency tones to be orthogonal. A cyclic prefix is formed by copying the tail samples from the time domain waveform and appending them to the front of the waveform. The time domain waveform with cyclic prefix is termed an OFDM symbol, and this OFDM symbol may be upconverted to a radio frequency (RF) and transmitted. An OFDM receiver may recover the timing and carrier frequency and then process the received samples through a fast Fourier transform (FFT). The cyclic prefix may be discarded and after the FFT, frequency domain information is recovered. The pilot symbols may be recovered to aid in channel estimation so that the data sent on the frequency tones can be recovered.
Several problems inherent in receiver design have been discussed by Razavi, “Design Considerations for Direct Conversion Receivers,” IEEE Trans. on Circuits and Systems—II: Analog and Digital Signal Processing, Vol. 44, No. 6, pp. 428-435, (June 1997). Some of these problems are related to receivers which use an intermediate frequency (IF) while others are related to direct conversion receivers which directly convert a received radio frequency (RF) signal to a baseband signal without IF conversion. IF receiver architectures involve a tradeoff between image rejection and adjacent channel rejection. Direct conversion receiver signals may be degraded by DC offset, I/Q mismatch, even-order distortion, flicker noise, and local oscillator leakage.
Flicker noise is produced by analog components such as resistors. The resulting noise is greatest at DC and decays with increasing frequency. It is often referred to as 1/f noise. Flicker noise causes the portion of a signal originally at the local oscillator frequency (fC−fIF) to be noisy. With a zero IF architecture (fIF=0) or direct conversion receiver, this noise is located in the center of the desired channel. However, if |fIF|>B/2+Bflicker, the flicker noise is effectively pushed out of the desired signal bandwidth. Here, B is the bandwidth of the desired channel, and Bflicker is the 1 sided bandwidth which captures the majority of the flicker noise.
Coupling from the local oscillator to the RF input of the mixer and between the low noise amplifier (LNA) of the receiver and the mixer produces a DC offset in the down converted signal. Like flicker noise, DC offset will affect the portion of the signal originally at the local oscillator frequency (fC−fIF). While it is possible to cancel the DC offset with a tracking loop in the digital domain, it is not necessarily convenient. For example, as the received signal strength changes, an automatic gain control (AGC) circuit will change the LNA gain, thereby changing the DC level. DC offset cancellation, therefore, must comprehend varying LNA gain. As with flicker noise, a simpler solution is to push the DC offset out of the bandwidth of the desired channel. This is accomplished when |fIF|>B/2+BDC, where BDC is the 1 sided bandwidth occupied by the DC offset.
Valkama et al., “Compensation of Frequency-Selective I/Q Imbalances in Wideband Receivers: Models and Algorithms,” 2001 IEEE Third Workshop on Signal Processing Advances in Wireless Communications (SPAWC '01), pp. 42-45 (March 2001) disclose a model of I/Q imbalance as shown at FIG. 2. The quadrature receiver of FIG. 2 receives a signal r(t) at lead 200. The in-phase branch includes mixer 202, low pass filter 204, and low pass filter imbalance 206, and produces in-phase signal z′I(t). Mixer 202 receives signal r(t) as well as the local oscillator signal cos(2πfCt) and provides an IF signal to low pass filter 204. The low pass filter selects a desired one-sided bandwidth. The quadrature branch includes mixer 212, low pass filter 214, and low pass filter imbalance 216, and produces quadrature signal z′Q(t). Mixer 212 receives signal r(t) as well as the local oscillator signal −g sin(2πfCt+φ) and provides an IF signal to low pass filter 214. The low pass filter 214 selects a desired one-sided bandwidth. Here, the gain and phase imbalance of the mixer is respectively modeled as “g” and “φ” in the local oscillator signal of the quadrature branch. Low pass filter imbalance in the model is represented by blocks 206 and 216 in the in-phase and quadrature branches, respectively. From this model, Valkama et al. derive a received frequency domain signal as shown at equations [1]-[3].Z′(f)=G1(f)Z(f)+G2(f)Z*(−f)  [1]G1(f)=(HI(f)+ge−jφHQ(f))/2  [2]G2(f)=(HI(f)−ge−jφHQ(f))/2  [3]
The first term G1(f)Z(f) of equation [1] is the desired signal. The second term G2(f)Z*(−f) of equation [1] is a mirror image aliasing term due to I/Q imbalance in the receiver. This aliasing term can cause severe interference with the desired signal. Valkama et al. disclose Adaptive Interference Cancellation (IC) and Multichannel Blind Deconvolution (MBD) as methods of reducing the interference.
While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements may be made by addressing some of the drawbacks of the prior art. In particular, the present inventors have recognized significant differences in interference due to the signal strength of adjacent channels. Accordingly, the preferred embodiments described below are directed toward these problems as well as improving upon the prior art.